1. Field of the Invention
The present invention relates to a profile element pipe for hydraulic bulging, a hydraulic bulging device using the profile element pipe, a hydraulic bulging method using the profile element pipe, and a hydraulically bulged product subjected to the hydraulic bulging.
2. Description of the Related Art
A hydraulic bulging has more merits as compared with other forming or forming methods. For example, since a profile element pipe can be hydraulically bulged to such an intricate configuration part having different cross-sections in the longitudinal direction of the product, machine parts, which require welding and joining in a conventional method, can be formed in one-piece. Further, since the hydraulic bulging generates work hardening over the entire hydraulically bulged portions, even if a soft element pipe is used, a product having high strength can be obtained.
Further, in the hydraulic bulging, the bulged product has small springback and a dimensional accuracy of the product is excellent (shape fixability is excellent). Thus a process for refining product dimension is not required and the omission of the process is effected.
In the hydraulic bulging, the above-mentioned excellent merits are appreciated and the hydraulic bulging has been particularly adopted as a production method of automotive parts in recent years.
Generally, in case that a pipe is formed by hydraulic bulging, a straight pipe having a uniform circular cross-section in the longitudinal direction of the pipe (hereinafter referred to as “straight element pipe”) is used as a material, and after this material was subjected to bending and stamping as a “pre-forming” hydraulic bulging is performed as a final working process. By taking such a series of working processes, a hydraulically bulged product can be manufactured by processing a straight element pipe to a product of a predetermined configuration.
FIGS. 1A and 1B are views showing a final working process of hydraulic bulging by which a product is obtained by using a conventional straight element pipe. As shown in FIGS. 1A and 1B, in the hydraulic bulging of the final process, a working liquid is injected into a straight element pipe P1 set in an upper die 1 and a lower die 2 through a filling hole 3 to load internal pressure. Further, in addition to the loading of internal pressure, the element pipe P1 is axially pressed (hereinafter referred to as “axial pressing or pushing”) from both ends of the pipe by pushing tools 4 and 5 also serving as sealing tools.
In the hydraulic bulging, the loading of internal pressure and the axial pressing are combined with each other so that a product P2 having various cross-sectional shapes is produced. It is noted that the pushing tools 4 and 5 serving also as sealing tools are connected to a hydraulic cylinder (not shown) and during hydraulic bulging its axial position or pressing force are controlled.
The pressing from a pipe end in the axial direction in the hydraulic bulging has such effects that a metal flow during bulging of an element pipe is promoted and an expansion limit of the element pipe is improved. Thus, in the hydraulic bulging, the axial pressing from the pipe end is an extremely important working process.
Specifically, when the hydraulic bulging is performed only by the loading of internal pressure without performing axial pressing or pushing, the wall thickness of the straight element pipe P1 is remarkably decreased with bulging of the straight element pipe P1. Therefore, the straight element pipe P1 ends up in rupture halfway through hydraulic bulging. Namely, it amounts to narrow a formable range (pipe expansion limit) of the straight element pipe P1.
Further, the hydraulic bulging has a problem attributable to a shape of the element pipe. As described above, even if an intricate configuration having different axial cross-sectional shapes can be obtained as one of the merits of the hydraulic bulging, the configuration of a worked product which can be obtained is limited.
For example, when the relationship of the increase ratio in a peripheral length (pipe expansion ratio)=[(outer peripheral length of a worked product at the portion/circumferential length of element pipe)−1] 100% is defined, the limit of increase ratio in a peripheral length (pipe expansion ratio) is at most 25% or so except for a region of the pipe end portion where axial pressing is effective, although the ratio depends on shape properties required for a bulged product or conditions (material, sheet thickness) of an element pipe to be used.
The hydraulic bulging cannot be performed beyond the limit of the increase ratio in the peripheral length (pipe expansion ratio). To increase a degree of freedom in a configuration design of a worked product and to obtain a worked product having a more intricate cross-sectional shape, it is necessary to contrive ways regarding the shape of an element pipe under a restricted condition of such an increase ratio in a peripheral length (pipe expansion ratio).
To deal with this problem, there has been proposed to use a substantially conical element pipe (hereinafter referred to as “tapered element pipe”) instead of a straight element pipe. Namely, by using the tapered element pipe, the increase ratio in a peripheral length due to working can be suppressed to a low level for parts which are difficult to be formed by using a straight element pipe, for example, for parts whose peripheral length varies in the axial direction, thereby enabling predetermined working shapes to be formed (see for example, Japanese Patent Application Publication No. 2001-321842, page 1, FIG. 2).
However, when hydraulic bulging is performed by using a tapered element pipe whose cross-sectional shape varies in the axial direction, in case of using a pressing or pushing tool for the straight element pipe shown in FIG. 1, it is found difficult to apply the axial pressing on the tapered element pipe.
FIG. 2 is a view explaining a problem, which arises when axial pressing with a conventional pressing tool for a straight element pipe was applied on a tapered element pipe. As shown in FIG. 2, the shaft pressing itself on a tapered element pipe TP1 cannot be applied on the large diameter side, although the axial pressing itself on the tapered element pipe TP1 can be applied on the small diameter side. However, as a pressing tool 4 advances into forms 1 and 2 with the axial pressing, insufficient restriction of inner and outer surfaces of the tapered element pipe TP1 by the pressing tool 4 side take places, thus likely leading up to seal leakage occurs.
FIGS. 3A to 3C are views explaining hydraulic bulging process using a conventional tapered element pipe, where FIG. 3A shows a state before processing, FIG. 3B shows a state before loading internal pressure, and FIG. 3C shows a state at the finish of processing.
In the conventional hydraulic bulging using the tapered element pipe TP1, as shown in FIGS. 3A to 3C, pressing tools 6 and 7, each having a tapered front end, are to be used. However, since axial pressing cannot be performed, hydraulic bulging is generally completed only by loading internal pressure without axial pressing. It is noted that TP2 in FIGS. 3A to 3C denotes a tapered element pipe subsequent to pipe-end pre-forming and TP3 denotes a hydraulically bulged product.
In the working process shown in FIGS. 3A to 3C, since the axial pressing of the tapered element pipe TP2 cannot be performed, the hydraulic bulging can be performed only in a limited range of forming to such a degree that rupture does not occur in a stage of hydraulic bulging, as described above. Therefore, in the hydraulic bulging, a merit of using the tapered element pipe is not in fact fully utilized.
Thus, in case where hydraulic bulging is performed using a tapered element pipe, a technological development, which enables pressing from the pipe end in the axial direction in addition to loading internal pressure on the element pipe, has been desired.
When hydraulic bulging is performed in a conventional tapered element pipe, there is a problem which arises when a hydraulically bulged product is joined with another member, other than the problem that axial pressing is difficult.
FIGS. 4A to 4C are views explaining a problem when a hydraulically bulged product having a rectangular cross-section is joined, wherein FIG. 4A shows a shape of a conventional hydraulically bulged product, and FIG. 4B shows a shape of a hydraulically bulged product according to the present invention, along with denoting inclinations of pipe end portions with respect to the axial direction of each worked product, and wherein FIG. 4C shows a configuration of a typical cross-section of the hydraulically bulged-products in FIG. 4A or 4B.
The hydraulically bulged product PT3 using a conventional tapered element pipe as a material is inclined in the pipe end portions by θ as shown in FIG. 4A. Thus, since accuracy cannot be ensured in welding and joining with another member, the joining with another member or the like is not easy.
Further, when an end of the pipe is socketed into another part and connected thereto, that is a socket connection, the accuracy cannot be ensured as well. Thus positioning of the tapered element pipe becomes difficult. Consequently, finishing process such as cutting off of very ends of hydraulically bulged product is required.